Process for Removing Carbon Dioxide From a Gas Stream using Desublimation

ABSTRACT

A process for removing carbon dioxide from a carbon dioxide containing gas stream is obtained through de-sublimation, vaporization, and liquefaction of various carbon dioxide-containing streams with little or no external refrigeration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/497,148, filed Jun. 15, 2011.

FIELD OF THE INVENTION

The present invention relates to a cost effective, efficient process forcapturing carbon dioxide from a gas stream using desublimation.

BACKGROUND

The emission of carbon dioxide from fossil fuel combustion is a growingsource of concern around the world. Numerous technologies have beendeveloped to address this problem. Research and development continue inthis area. One avenue of research has concentrated on the capture ofcarbon dioxide from flue gas and other gas streams. Some of the majortechnologies utilized have included amine washes, physical adsorptiontechnologies, cryogenic technologies (carbon dioxide liquefaction) andsuch. However, these technologies involve significant additionalinvestment and operating costs for industrial plants. For example, inthe case of coal power plants a resulting increase of the cost ofelectricity in the range of 4 to 5 U.S. cents/kWh is expected.

One possible alternative to traditional capture solutions which isdescribed in literature is called anti-sublimation. This processbasically involves separating carbon dioxide from a flue gas by coolingthe flue gas to turn the carbon dioxide into a solid (de-sublimation orcryo-condensation of carbon dioxide). Indeed, at such low carbon dioxidepartial pressure (<5.11 atmosphere), the carbon dioxide will be directlychanged from a gas phase to a solid phase. There exist two mainapproaches to implement such a process. The first of these approachesinvolves de-sublimation at atmospheric or very low pressure. With thisfirst approach, a significant external refrigeration loop is required toperform such a cooling (indirect de-sublimation). The second approach isde-sublimation at higher pressure by expansion with solid formation(direct de-sublimation).

De-sublimation has been described extensively in the literature. See,for example, Cost Analysis of CO₂ Capture by Antisublmation Applied ForCoast Fired Boilers, CCT 2007, 16 May 2007; FR 2851936; FR 2820052; FR2894838; WO 05082493; JP 2000317302; WO 09047341; FR 2867092; WO09070785; JP 2004286348; WO 06062595; U.S. 6082133; EP 1754695; WO08095258; AU 2003201393; AU 2003270123; and WO 09070785.

With regard to the two approaches noted, the first approach is thesimplest approach but has several drawbacks. The second approach hasshown to be more attractive in terms of energy efficiency, for two mainreasons: direct cooling can improve efficiency and there are lesspressure drops because of the higher pressure. However, there is nomention in the literature of a process that would simultaneously work ata relatively low pressure and require no external refrigeration. Indeed,although the need of external refrigeration is drastically reduced bythe second approach, a small external refrigeration loop is usuallyneeded. This external loop adds equipment and decreases the overallefficiency of the process.

Accordingly, there exists a need to reduce the cost of carbon dioxidecapture from flue gas through improved efficiencies and reduced capitalexpenditures.

SUMMARY OF THE INVENTION

The present invention relates to a highly efficient auto-refrigeratedprocess for de-sublimation of a flue gas. In this process, the flue gasis first compressed to a medium range. This compressed flue gas is thencooled before being subjected to a partial desublimation to producesolid carbon dioxide and cold medium pressure flue gas. The solid carbondioxide is then separated from the cold medium pressure flue gas. Thecold medium pressure flue gas is then introduced into an expansionturbine where additional solid carbon dioxide is formed and the pressureof the flue gas is dropped to a low range. This additional solid carbondioxide is then separated from the low pressure flue gas to produce acarbon dioxide depleted flue gas. The low pressure carbon dioxidedepleted flue is gas heated to recover sensible heat. The solid carbondioxide is split into at least two streams with part of the solid carbondioxide being brought to or above triple point pressure and liquefied.Sensible heat of the solid and the liquid is recovered as well as fusionheat of the solid when liquefied. The remaining part of the solid isbrought to lower pressure (sub-atmospheric pressure) and vaporized atlow pressure. Sublimation heat of the low pressure carbon dioxide isrecovered to heat exchange with de-sublimating flue gas. Gaseoussub-atmospheric pressure carbon dioxide is then compressed bysub-atmospheric compression means. A portion of this can be compressedto a pressure high enough to be condensed at ambient temperature andthen pumped to final pressure. The remaining part can be compressed toan intermediate pressure and condensed at a lower pressure by heatexchange with the rest of the process.

The invention is also directed to a process for removing carbon dioxidefrom a carbon dioxide containing gas stream, said process comprising thesteps of: a) compressing the carbon dioxide containing gas stream to apressure that ranges from about 1.1 bar absolute to about 20.0 barabsolute to produce a compressed gas stream; b) routing the compressedgas stream through a main heat exchange system comprising one or moremain heat exchangers where the temperature of the compressed gas streamis reduced to a point that a portion of the carbon dioxide in thecompressed gas stream is transformed into solid carbon dioxide with theremainder of the carbon dioxide remaining in gaseous form; c) separatingthe solid carbon dioxide from the compressed cooled gas stream to form astream of solid carbon dioxide and a cooled partially carbon dioxidedepleted gas stream; d) introducing the cooled partially carbon dioxidedepleted gas stream into an expansion turbine in order to transformadditional carbon dioxide within the gas stream into additional solidcarbon dioxide; e) separating the additional solid carbon dioxide fromthe cooled partially carbon dioxide depleted gas stream to form anadditional stream of solid carbon dioxide and an expanded carbon dioxidedepleted gas stream; f) routing the expanded carbon dioxide depleted gasstream back through the main heat exchange system where the expandedcarbon dioxide depleted gas stream is heated to a temperature thatranges from about −20° C. to about 50° C. to recover sensible heat andthen venting the carbon dioxide depleted gas stream that is withdrawnfrom the heat exchanger; and g) using the streams of solid carbondioxide produced to reduce the temperature of the compressed gas streamin step b) by: i) subjecting a portion of the solid carbon dioxide to apressure that is equal to or greater than the triple point pressure ofcarbon dioxide in a first vessel in communication with the main heatexchange system to produce a carbon dioxide liquid stream and recoveringfusion heat and sensible heat; ii) vaporizing a portion of the solidcarbon dioxide in a second vessel in communication with the main heatexchange system at sub-atmospheric pressure to produce a sub-atmosphericpressure carbon dioxide gas and recovering at least sublimation heat;and iii) compressing the sub-atmospheric pressure carbon dioxide gasobtained to produce a compressed carbon dioxide stream.

The invention is also directed to a process for removing carbon dioxidefrom a carbon dioxide containing gas stream, said process comprising thesteps of: A) routing the carbon dioxide containing gas stream through amain heat exchange system comprising one or more heat exchangers wherethe temperature of the gas stream is reduced to a point that a portionof the carbon dioxide in the gas stream is transformed into solid carbondioxide with the remainder of the carbon dioxide remaining in gaseousform, the carbon dioxide containing gas stream having a pressure fromabout 0.8 to 3.0 bar absolute; B) separating the solid carbon dioxidefrom the cooled carbon dioxide containing gas stream to form a stream ofsolid carbon dioxide and a cooled partially carbon dioxide depleted gasstream; C) routing the cooled partially carbon dioxide depleted gasstream back through the main heat exchange system where the cooledpartially carbon dioxide depleted gas stream is heated to a temperaturethat ranges from about −20° C. to about 50° C. to recover sensible heatand then venting the carbon dioxide depleted gas stream that iswithdrawn from the heat exchanger; and D) using the stream of solidcarbon dioxide produced to reduce the temperature of the carbon dioxidecontaining gas stream in step b) by: i) subjecting a portion of thesolid carbon dioxide to a pressure that is equal to or greater than thetriple point pressure of carbon dioxide in a first vessel incommunication with the main heat exchange system to produce a carbondioxide liquid stream and recovering fusion heat and sensible heat; ii)vaporizing a portion of the solid carbon dioxide in a second vessel incommunication with the main heat exchange system at sub-atmosphericpressure to produce a sub-atmospheric pressure carbon dioxide gas andrecovering at least sublimation heat; and iii) compressing thesub-atmospheric pressure carbon dioxide gas obtained to produce acompressed carbon dioxide stream.

The invention may include one or more of the following aspects:

the portion of solid carbon dioxide subjected to a pressure that isequal to or greater than the triple point pressure of carbon dioxide toproduce a carbon dioxide liquid stream is obtained from step b).

at least a portion of the solid carbon dioxide vaporized atsub-atmospheric pressure to recover at least the sublimation heat andproduce a sub-atmospheric pressure carbon dioxide gas is obtained fromstep d).

prior to step g), the solid carbon dioxide of step b) and step d) iscombined for use in step g).

the carbon dioxide containing gas stream is a flue gas stream.

in step e), the vaporization of the solid carbon dioxide is carried outwith at least two different sub-atmospheric pressures.

the vaporization of the solid carbon dioxide with at least two differentsub-atmospheric pressures is performed in at least two independentvessels with each independent vessel corresponding to a differentsub-atmospheric pressure.

the vessels are in thermal communication with the main heat exchangesystem through a series of pipes that run through the main heat exchangesystem and the various vessels.

the recovery of the sublimation heat is carried out by circulating oneor more fluids between the main heat exchange system and the vessels torecover cold from the phase changes in the vessels and release this coldin the main heat exchange system.

the one or more fluids are circulated by pumping the one or more fluidsor by thermo-siphoning.

the one or more fluids are selected from CF₄, NF₃, C₂H₆

the compressed carbon dioxide stream is further treated to obtain thedesired carbon dioxide product.

the carbon dioxide liquid stream is further treated to obtain thedesired carbon dioxide product.

from 90 to 99% of the carbon dioxide in the carbon dioxide containinggas stream is separated by de-sublimination with no externalrefrigeration cycle.

the latent heat of fusion for the liquefaction of the solid carbondioxide in step g i is obtained from condensation of gaseous carbondioxide in the stream of compressed carbon dioxide stream obtained fromstep g iii.

no external source of refrigeration is utilized.

the carbon dioxide containing stream is compressed prior to being routedinto the main heat exchanger in order to have a pressure from about 1.1to 3.0 bar absolute.

the carbon dioxide containing gas stream is a flue gas stream.

in step D), the vaporization of the solid carbon dioxide is carried outwith at least two different sub-atmospheric pressures.

the latent heat of fusion for the liquefaction of the solid carbondioxide in step D i is obtained from condensation of gaseous carbondioxide in the stream of compressed carbon dioxide stream obtained fromstep D iii.

DESCRIPTION OF THE FIGURES

FIG. 1 presents a detailed example of such a process that have beencalculated using process modeling tools and other carbon dioxide gasproperties (solid phase properties, sublimation, fusion . . . ).

FIG. 2 presents the associated main heat exchanger diagram (temperatureversus heat flow).

DETAILED DESCRIPTION OF THE INVENTION

The present process relates to a more efficient process for the removalof carbon dioxide from a carbon dioxide containing stream. By capturingthe energy associated with the transition of solid carbon dioxide toeither a liquid or vapor, it is possible to eliminate or at leastsignificantly minimize the use of the external cooling systems such asadditional refrigerant loops including compressors as well as thecooling means for compressed refrigerant that have previously been usedin the prior art systems of carbon dioxide removal. The process may becarried out in a direct manner in which the use of the prior artexternal cooling systems are eliminated or in an indirect manner inwhich the prior art external cooling systems are minimized (used only tobolster the process).

With regard to the carbon dioxide containing gas streams that may betreated utilizing either embodiment of the present invention, suchstreams may be provided from a variety of sources, including but notlimited to, flue gas streams, steam methane reforming syngas streams,pressure swing adsorption tail gas streams, gasification streams, carbondioxide contaminated natural gas streams, metal treatment furnacestreams, or refinery streams. The present invention is considered to beparticularly advantageous with regard to the treatment of carbon dioxidecontaining streams which are considered to have low concentrations ofcarbon dioxide. As used herein, the phrase “low concentrations of carbondioxide” refers to those carbon dioxide containing streams that haveless than 50% carbon dioxide (as measured by molar fraction), especiallythose carbon dioxide containing streams that have equal to or less than30%. One particularly preferred carbon dioxide containing gas fortreatment with the present process is a flue gas stream having less thanor equal to 30% carbon dioxide.

The first embodiment of the present invention involves the direct mannerof removing carbon dioxide from carbon dioxide containing streamswithout the need to include an external cooling system. The first stepof the process involves compressing a carbon dioxide containing gasstream obtained from any of the sources defined hereinbefore to apressure that ranges from about 1.1 bar to about 20.0 bar absolute inorder to obtain a compressed gas stream. In one alternative, thepressure may range from about 2.0 bar absolute to about 16.0 barabsolute. In a still further alternative, the pressure may range fromabout 3.0 bar absolute to about 10.0 bar absolute. The actual level ofcompression needed may be achieved by any means known in the art such asby utilizing a compressor. In addition, while the term “a compressor” isutilized, the degree of compression may also be achieved by utilizingmore than one compressor (multiple compressors in series). Thiscompression can be isothermal (several stages with inter-stage cooling)or adiabatic (no or less inter-stage cooling but heat recovery of theheat from the hot pressurized gas stream in a steam cycle). Also, in theinstance that the carbon dioxide containing gas is already within thiscompression level (where the source from which the carbon dioxidecontaining gas is obtained produces a carbon dioxide containing gas thatis already at a pressure level as noted), it may be possible toeliminate this step.

Once the compressed gas stream is achieved, this gas stream is routedthrough a main heat exchange system where the temperature of thecompressed gas stream is reduced to a point that a portion of the carbondioxide in the compressed gas stream is transformed into solid carbondioxide while the remainder of the carbon dioxide in the carbon dioxidecontaining gas stream remains in gaseous form. A variety of differenttypes of heat exchangers for carrying out processes such as those of thepresent invention are commercially available. Such heat exchangers aretypically referred to as multiple zone heat exchangers. Note that thepresent invention is not meant to be limited with regard to theparticular type of heat exchanger utilized in the main heat exchangesystem provided that the heat exchange system has multiple zones.Alternatively, in the present invention the function of main heatexchange system could be carried out by several heat exchangers, inparallel and/or in series. In particular, those skilled in the art willrecognize that it can be advantageous to separate heat exchange inseveral heat exchangers to simplify the heat exchangers as well asseparate the different functions requiring different technologies.Regardless of the specific type of main heat exchange system utilized,in the case of multiple heat exchange zones or multiple heat exchangers,the multiple heat exchange zones or multiple heat exchangers may bethermally integrated with one another. This means that they areintentionally designed to achieve heat transfer between one another.

As noted, the temperature of the compressed gas stream is reduced to apoint that a portion of the carbon dioxide contained in the compressedgas stream is transformed into solid carbon dioxide. More specifically,this transformation from a gas to a solid with no intervening liquidform is typically referred to as cryo-condensation, anti-sublimation orde-sublimation. Sublimation, the opposite of deposition orde-sublimation refers to the change of carbon dioxide from a solid to agas with no intervening liquid form. Note that at temperatures below−56.6° C. and pressures below 5.11 atm (the triple point),sublimation/de-sublimation occurs. At atmospheric pressure, thistemperature is −78.5° C.

De-sublimation occurs within the main heat exchange system as the gasstream passes through the main heat exchange system. Once thede-sublimation occurs, the compressed gas stream that exits the mainheat exchange system includes some carbon dioxide that is in solid formand some carbon dioxide that is in gaseous form. Accordingly, the nextstep in the process is to separate the solid carbon dioxide from thecompressed cooled gas stream thereby forming a stream of solid carbondioxide and a cooled partially carbon dioxide depleted gas stream. Withregard to the present invention, the phrase “solid carbon dioxide”refers to the carbon dioxide produced in solid form due to thede-sublimation. This solid carbon dioxide will typically appear in formlike a powdering of snow. In addition, the phrase “stream of solidcarbon dioxide” refers to the individual bits or particles of carbondioxide that when separated from the compressed cooled gas stream areconsidered to form a “stream” of the solid carbon dioxide particles. Thecompressed gas stream that includes some carbon dioxide that is in solidform can be separated into a stream of solid carbon dioxide and a cooledpartially carbon dioxide depleted gas by any separation method known inthe art. Typically, as the compressed gas stream that includes somecarbon dioxide in solid form exits the main heat exchange system, thecompressed gas stream is passed through a cyclone in order to separatethe solids from the gases. In an alternative case, the separation can beperformed simultaneously with the de-sublimation. For instance, solidcarbon dioxide can be allowed to settle or fall out of the gas stream asthe gas stream passes along thereby resulting in the separation (see forexample WO 2010/107820).

After the stream of solid carbon dioxide and the cooled partially carbondioxide depleted gas are obtained, the cooled partially carbon dioxidedepleted gas stream is introduced into an expansion turbine. Anyexpansion turbine known in the art may be utilized in the presentprocess to transform at least a portion of the remainder of the carbondioxide found within the cooled partially carbon dioxide depleted gasstream into additional solid carbon dioxide. In addition, those skilledin the art will recognize that a series of expansion turbines may alsobe utilized to carry out this transformation. The expansion turbinecould also have a specific design in order to meet the specificrequirements of handling solid formation. As the cooled partially carbondioxide depleted gas stream is passed through the expansion turbine, thegas stream expands thereby resulting in a cooling of the gas stream andthe transformation of part of the carbon dioxide in the gas stream froma gaseous state into a solid state. While the phrase “remainder of thecarbon dioxide” is utilized with regard to the description of theprocess, it should be noted that the process of the present invention,while ultimately seeking to remove close to 100% of the carbon dioxidepresent in the original carbon dioxide containing gas stream, will oftenresult in some of the gaseous form carbon dioxide remaining in theresulting expanded carbon dioxide depleted gas stream. Typically, it ispossible to remove from 90 to 99% of the carbon dioxide present in theoriginal carbon dioxide containing gas stream thereby resulting in anexpanded carbon dioxide depleted gas stream which actually contains from0.1 to 10% (molar percent) of carbon dioxide with the preference beingthat the amount remaining be between about 0.1% and about 2.0%.

The separation of the additional solid carbon dioxide from the cooledpartially carbon dioxide depleted gas stream to form an additionalstream of solid carbon dioxide and an expanded carbon dioxide depletedgas stream can be carried out in the same manner as noted above withregard to the separation of the solid carbon dioxide from the compressedcooled gas stream to form a stream of solid carbon dioxide and a cooledpartially carbon dioxide depleted gas stream. More specifically, theadditional solid carbon dioxide from the cooled partially carbon dioxidedepleted gas stream utilization any separation means that are known inthe art for separating solids from gases including the use of a cyclone.The expansion is carried out to achieve a gas stream that is down to apressure from about 1.0 bar absolute to about 3.0 bar absolute,preferably close to atmospheric pressure, more specificallyapproximately 1.3 bar absolute. Note the preference of 1.3 bar absoluteis to compensate for any pressure drops occurring after the turbine inorder to vent as close as possible to atmospheric pressure.

The expanded carbon dioxide depleted gas stream that is obtained is thenrouted back through the main heat exchange system where the expandedcarbon dioxide depleted gas stream is heated to a temperature thatranges from about −20° C. to about 50° C. in order to recover sensibleheat. As used herein, the term “sensible heat” refers to the heat orenergy produced due to the change in temperature. Once the carbondioxide depleted gas stream is passed through the main heat exchangesystem, it is withdrawn and vented to the atmosphere.

The streams of solid carbon dioxide that are produced are then utilizedto reduce the temperature of the compressed gas stream as notedhereinbefore. This is accomplished by first subjecting a portion of thesolid carbon dioxide obtained to a pressure that is equal to or greaterthan the triple point pressure of carbon dioxide in a first vessel toproduce a carbon dioxide liquid stream and recovering fusion heat andsensible heat. Note that the first vessel can be any vessel known in theart that is sufficient to allow for the subjecting of the solid carbondioxide to the noted pressure. Accordingly, as used herein with regardto the present process, the term “vessel” is not meant to be restrictiveand is simply utilized to denote where the solid carbon dioxide issubjected to the conditions noted. Non-limiting examples of such vesselsinclude, but are not limited to any kind of storage tanks (vertical,horizontal or spherical). The first vessel is configured in such amanner that it is in communication with the main heat exchanger in orderto recover fusion heat and sensible heat. While this communication canbe in any manner known in the art, typically the communication will bethrough a series of pipes that run through the main heat exchange systemand the first vessel. More specifically, the series of pipes will runthrough at least one of the at least one heat exchanger of the main heatexchange system and through the first vessel. As sublimation occurs inthe first vessel (the transformation of the solid carbon dioxide togaseous carbon dioxide), the sensible heat and the fusion heat arerecovered by circulating one or more fluids through the pipes betweenthe main heat exchange system and the first vessel. This allows for therecovery of cold due to the phase changes in the first vessel and therelease of this cold in the main heat exchange system.

In the next step of the process, a portion of the solid carbon dioxideis vaporized in a second vessel that is also in communication with themain heat exchange system. This vaporization occurs at sub-atmosphericpressure to allow for the production of a sub-atmospheric pressurecarbon dioxide gas and the recovery of at least sublimation heat. In analternative schematic, the vaporization of the solid carbon dioxide iscarried out with at least two different sub-atmospheric pressures. Whenthe vaporization of the solid carbon dioxide is carried out with atleast two different sub-atmospheric pressures, the vaporization iscarried out in at least two independent vessels with each independentvessel corresponding to a different sub-atmospheric pressure.

As noted with regard to the first vessel, the second vessel, as well asany other vessels that might be utilized, are in communication with themain heat exchange system through a series of pipes that run through themain heat exchange system and the various vessels. By circulating one ormore fluids through these pipes between the main heat exchange systemand the various vessels it is possible to recover cold from the phasechanges in the vessels and release this cold in the main heat exchangesystem. The one or more fluids in the pipes are typically circulatedusing any method that is known in the art, for example, by pumping theone or more fluids or by thermo-siphoning. Typically, the one or morefluids are selected from CF₄, NF₃, C₂H₆ although other fluids that areknown in the art for such purposes may be utilized.

The sub-atmospheric pressure carbon dioxide gas obtained is compressedto produce a compressed carbon dioxide stream. This compressed carbondioxide stream is further treated by any methods known in the art toobtain the desired carbon dioxide product. In addition, the carbondioxide liquid stream is further treated to obtain the desired carbondioxide product. For example, the desired product could be: under liquidconditions in which case the liquid would require limited furthertreatment and the gas would require at least compression and thencooling; or under super-critical conditions where the liquid wouldrequire at least further compression and/or pumping and the gas wouldrequire at least further compression and cooling. Liquefaction of thiscompressed carbon dioxide stream may also be accomplished through heatexchange with the above-described solid carbon dioxide that isliquefied. In this manner, the heat of vaporization is exchanged withthe heat of fusion to condense carbon dioxide into liquid form from thecompressed carbon dioxide stream and to liquefy the solid carbondioxide.

In the second embodiment of the present process, the carbon dioxide isremoved from the carbon dioxide containing streams using an indirectmanner. According to this embodiment, the use of an external coolingsystem is minimized, if not eliminated. The first step of the processinvolves routing the carbon dioxide containing gas stream though a mainheat exchange system comprising one or more heat exchangers where thetemperature of the carbon dioxide containing gas stream is reduced to apoint that a portion of the carbon dioxide in the carbon dioxidecontaining gas stream is transformed into solid carbon dioxide (asdefined hereinbefore) while the remainder of the carbon dioxide in thecarbon dioxide containing gas stream remains in gaseous form. Note thatwhen the carbon dioxide containing gas stream is introduced into themain heat exchange system, it is introduced at a pressure that rangesfrom about 1.1 bar to about 3.0 bar absolute, preferably from about 1.1bar absolute to about 2.0 bar absolute. In those instances where thecarbon dioxide containing gas stream to be utilized is not within thispressure range, the carbon dioxide containing gas stream is firstcompressed in one or more compressors to achieve this level of pressureas described hereinbefore with regard to the first embodiment.

As noted hereinbefore, a variety of different types of heat exchangersare available for the main heat exchange system of the processes of thepresent invention such as multiple zone heat exchangers in paralleland/or series. This embodiment, as with the previous embodiment, is notmeant to be limited with regard to the particular type of heat exchangerutilized provided that the heat exchanger has multiple zones. Themultiple zones may be thermally integrated with one another. As thecarbon dioxide containing gas stream is routed through the main heatexchange system, the temperature of the carbon dioxide containing gasstream is reduced to a point that a portion of the carbon dioxidecontained in the compressed gas stream is transformed into solid carbondioxide with no intervening liquid form. Once the de-sublimation occurs,the gas stream that exits the main heat exchange system includes somecarbon dioxide that is in solid form and some carbon dioxide that is ingaseous form. Accordingly, the next step in the process is to separatethe solid carbon dioxide from the cooled carbon dioxide containing gasstream thereby forming a stream of solid carbon dioxide and a cooledpartially carbon dioxide depleted gas stream. The carbon dioxidecontaining gas stream that includes some carbon dioxide that is in solidform can be separated into a stream of solid carbon dioxide and a cooledpartially carbon dioxide depleted gas stream by any separation methodknown in the art including through a cyclone or by allowing the solid tosettle or fall out of the gas stream after the gas stream exits the mainheat exchange system.

After the stream of solid carbon dioxide and the cooled partially carbondioxide depleted gas are obtained, the cooled partially carbon dioxidedepleted gas stream is then routed back through the main heat exchangesystem where the cooled partially carbon dioxide depleted gas stream isheated to a temperature that ranges from about −20° C. to about 50° C.in order to recover sensible heat as defined hereinbefore. Once thecarbon dioxide depleted gas stream is passed through the main heatexchange system, it is withdrawn and vented to the atmosphere.

The stream of solid carbon dioxide that is produced is used to reducethe temperature of the carbon dioxide gas stream as noted hereinbeforeby first subjecting a portion of the solid carbon dioxide obtained to apressure that is equal to or greater than the triple point pressure ofcarbon dioxide in a first vessel as defined hereinbefore to produce acarbon dioxide liquid stream and recovering fusion heat and sensibleheat. The first vessel is configured in such a manner that it is incommunication with the main heat exchange system in order to recoverfusion heat and sensible heat, typically through a series of pipes thatrun through the main heat exchange system and the first vessel. Asmelting occurs in the first vessel (the transformation of the solidcarbon dioxide to liquid carbon dioxide), the sublimation heat and thefusion heat are recovered by circulating one or more fluids through thepipes between the main heat exchange system and the first vessel therebyallowing for the recovery of cold due to the phase changes in the firstvessel and the release of this cold in the main heat exchange system.Alternatively, the solid carbon dioxide may be liquefied through heatexchange with a gaseous carbon dioxide stream in a separate heatexchange system. As the solid carbon dioxide receives the heat of fusionfrom the gaseous carbon dioxide stream, the gaseous carbon dioxide iscondensed to liquid.

In the next step of the process, a portion of the solid carbon dioxideis vaporized at sub-atmospheric pressure in a second vessel that is alsoin communication with the main heat exchange system. This allows for theproduction of a sub-atmospheric pressure carbon dioxide gas and therecovery of at least sublimation heat. In an alternative schematic, thevaporization of the solid carbon dioxide is carried out with at leasttwo different sub-atmospheric pressures in at least two independentvessels with each independent vessel corresponding to a differentsub-atmospheric pressure.

The various vessels are in communication with the main heat exchangesystem through a series of pipes that run through the main heat exchangesystem and the various vessels. By circulating one or more fluidsthrough these pipes between the main heat exchange system and thevarious vessels it is possible to recover cold from the phase changes inthe vessels and release this cold in the main heat exchange system. Theone or more fluids in the pipes are typically circulated using anymethod that is known in the art, for example, by pumping the one or morefluids or by thermo-siphoning. Typically, the one or more fluids areselected from CF₄, NF₃, C₂H₆ although other fluids that are known in theart for such purposes may be utilized.

In this embodiment, the sub-atmospheric pressure carbon dioxide gasobtained is compressed to produce a compressed carbon dioxide stream.This compressed carbon dioxide stream is further treated by any methodsknown in the art to obtain the desired carbon dioxide product. Inaddition with regard to this embodiment, the carbon dioxide liquidstream is further treated to obtain the desired carbon dioxide product.

The first embodiment of the present process will be further describedwith regard to FIG. 1. Note that this figure is in no way meant to belimiting with regard to the process of the present invention. In FIG. 1,a flue gas stream (1) is first compressed in a compressor (2) to apressure between about 1.1 bar absolute and 20.0 bar absolute,preferably to approximately 6.0 bar absolute. This compression can beisothermal (several stages with inter-cooling; not shown) or adiabatic(no or less inter-cooling but heat recovery of the heat from the hotpressurized flue gas in the steam cycle). After cooling to ambientconditions the pressurized flue gas stream (1.1) enters a main heatexchanger (3) and is cooled down to approximately −90° C. therebyforming solid carbon dioxide within the gaseous flue gas stream (1.1).This cryocooled gas stream (1.2) that comprises solid carbon dioxide,gaseous carbon dioxide and the remaining components of the flue gasstream then exists the main heat exchanger (3) and the part of thecarbon dioxide stream that has been de-sublimated (5) is separated fromthe rest of the flue gas stream (4). This cold partially carbon dioxidedepleted flue gas stream (4) is then expanded through an expansionturbine (6) down to close to atmospheric pressure (approximately 1.3 barabsolute) where more solid carbon dioxide is formed As the stream exitsthe expansion turbine (6), the additional solid carbon dioxide (8) isseparated from the remaining portion of gas stream (4) to form anexpanded carbon dioxide depleted gas stream (10). The carbon expandedcarbon dioxide depleted gas stream (10) is then routed back through themain heat exchanger (3) to be heated in the main heat exchanger (3) inorder to recover cold before venting the gas stream (11) at close toambient temperature.

The first solid carbon dioxide stream (5) is entered in a first vessel(7) that is maintained at a pressure close to the triple point of carbondioxide. By direct or indirect heat exchange, part of the first solidcarbon dioxide stream (5) is heated and liquefied in this vessel (7) toprovide cold in the line (7.1) that runs through the main heat exchanger(3). The resulting liquid is then pumped (21) to recover sensible heatfrom the liquid up to about 10° C. in the line (7.1) of the main heatexchanger (3). The resulting stream (23) is mixed with other streams tobe the final carbon dioxide product. The remaining part of the solid isentered in the second vessel (9) after it's pressure has been reduced.

The second solid carbon dioxide stream (8) is sent to the second vessel(9) after reducing the pressure down to about 585 mbar absolute. Bydirect or indirect heat exchange, part of the solid carbon dioxidestream (8) is heated and vaporized in this vessel (9) to provide cold inthe line (9.1) of the main heat exchanger (3). The resulting gas stream(11) is recovered at −55° C. for compression. The remaining part of thesolid carbon dioxide stream (8) is entered in a third vessel (10) afterreducing the pressure of the remaining part of the solid carbon dioxidestream (8). The third vessel (10) is maintained at a pressure ofapproximately 375 mbar absolute. By direct or indirect heat exchange,part of the solid carbon dioxide stream (8) is heated and vaporized inthis vessel (10) to provide cold in the line (10.1) of the main heatexchanger(3). The resulting gas stream (12) is recovered at −55° C. forcompression in vacuum pressuring equipment (13). It is then mixed withgas stream 11 and compressed again by one or several stages ofadditional compression including vacuum pressuring equipment again (14).Part of the partially compressed carbon dioxide stream (16) is sent at apressure of approximately 6.0 bar absolute in the line (16.1) of themain heat exchanger (3) in ordered to be condensed and recovered as aliquid in a separator (17). The resulting liquid is then pumped (22) torecover sensible heat from the liquid up to 10° C. in the line (22.1) ofthe main heat exchanger (3). The resulting stream (19) is mixed withother streams to be the final carbon dioxide product.

The remaining part of the partially compressed carbon dioxide (24) isfurther compressed (15) up to 60 bar and then condensed at ambientconditions (18), typically using available cooling water. It is thenmixed with streams 19 and 23 to be pumped to its final pressure (forexample to 150 bar absolute for pipeline transport).

The above is only one of the many examples of the present process usingthe described invention. For instance, the use of a single line for heatexchange may not be convenient, especially in terms of handling solids.In addition, the process describes the heat exchange as occurring in onemain heat exchanger but the present process is also contemplated to beformulated to be divided in one de-sublimating heat exchanger andseveral other heat recovery heat exchangers.

The heat exchange between the three vessels can be done directly orindirectly. For example a fluid could be circulated between the saidvessels and the main heat exchanger(s). This fluid could be for examplea liquid circulated by a pump or an evaporating liquid (vaporization inthe heat exchanger and condensation in the vessel) also circulated by apump under the liquid phase. This fluid could be any fluid able tocondense/vaporize at the desired pressures and temperatures and could beone different fluid for each vessel.

1. A process for removing carbon dioxide from a carbon dioxidecontaining gas stream, said process comprising the steps of: a.compressing the carbon dioxide containing gas stream to a pressure thatranges from about 1.1 bar absolute to about 20.0 bar absolute to producea compressed gas stream; b. routing the compressed gas stream through amain heat exchange system comprising one or more main heat exchangerswhere the temperature of the compressed gas stream is reduced to a pointthat a portion of the carbon dioxide in the compressed gas stream istransformed into solid carbon dioxide with the remainder of the carbondioxide remaining in gaseous form; c. separating the solid carbondioxide from the compressed cooled gas stream to form a stream of solidcarbon dioxide and a cooled partially carbon dioxide depleted gasstream; d. introducing the cooled partially carbon dioxide depleted gasstream into an expansion turbine in order to transform additional carbondioxide within the gas stream into additional solid carbon dioxide; e.separating the additional solid carbon dioxide from the cooled partiallycarbon dioxide depleted gas stream to form an additional stream of solidcarbon dioxide and an expanded carbon dioxide depleted gas stream; f.routing the expanded carbon dioxide depleted gas stream back through themain heat exchange system where the expanded carbon dioxide depleted gasstream is heated to a temperature that ranges from about −20° C. toabout 50° C. to recover sensible heat and then venting the carbondioxide depleted gas stream that is withdrawn from the heat exchanger;g. using the streams of solid carbon dioxide produced to reduce thetemperature of the compressed gas stream in step b) by: i) subjecting aportion of the solid carbon dioxide to a pressure that is equal to orgreater than the triple point pressure of carbon dioxide in a firstvessel in communication with the main heat exchange system to produce acarbon dioxide liquid stream and recovering fusion heat and sensibleheat; ii) vaporizing a portion of the solid carbon dioxide in a secondvessel in communication with the main heat exchange system atsub-atmospheric pressure to produce a sub-atmospheric pressure carbondioxide gas and recovering at least sublimation heat; and iii)compressing the sub-atmospheric pressure carbon dioxide gas obtained toproduce a compressed carbon dioxide stream.
 2. The process of claim 1,wherein the portion of solid carbon dioxide subjected to a pressure thatis equal to or greater than the triple point pressure of carbon dioxideto produce a carbon dioxide liquid stream is obtained from step b). 3.The process of claim 1, wherein at least a portion of the solid carbondioxide vaporized at sub-atmospheric pressure to recover at least thesublimation heat and produce a sub-atmospheric pressure carbon dioxidegas is obtained from step d).
 4. The process of claim 1, wherein priorto step g), the solid carbon dioxide of step b) and step d) is combinedfor use in step g).
 5. The process of any claim 1, wherein the carbondioxide containing gas stream is a flue gas stream.
 6. The process ofclaim 1, wherein in step e), the vaporization of the solid carbondioxide is carried out with at least two different sub-atmosphericpressures.
 7. The process of claim 6, wherein the vaporization of thesolid carbon dioxide with at least two different sub-atmosphericpressures is performed in at least two independent vessels with eachindependent vessel corresponding to a different sub-atmosphericpressure.
 8. The process of claim 7, wherein the vessels are in thermalcommunication with the main heat exchange system through a series ofpipes that run through the main heat exchange system and the variousvessels.
 9. The process of claim 8, wherein the recovery of thesublimation heat is carried out by circulating one or more fluidsbetween the main heat exchange system and the vessels to recover coldfrom the phase changes in the vessels and release this cold in the mainheat exchange system.
 10. The process of claim 9, where the one or morefluids are circulated by pumping the one or more fluids or bythermo-siphoning.
 11. The process of claim 10, wherein the one or morefluids are selected from CF₄, NF₃, C₂H₆
 12. The process of claim 1,wherein the compressed carbon dioxide stream is further treated toobtain the desired carbon dioxide product.
 13. The process of claim 1,wherein the carbon dioxide liquid stream is further treated to obtainthe desired carbon dioxide product.
 14. The process of claim 1, whereinfrom 90 to 99% of the carbon dioxide in the carbon dioxide containinggas stream is separated by de-sublimination with no externalrefrigeration cycle.
 15. The process of claim 1, wherein the latent heatof fusion for the liquefaction of the solid carbon dioxide in step g iis obtained from condensation of gaseous carbon dioxide in the stream ofcompressed carbon dioxide stream obtained from step g iii.
 16. Theprocess of claim 1, wherein no external source of refrigeration isutilized.
 17. A process for removing carbon dioxide from a carbondioxide containing gas stream, said process comprising the steps of: a.routing the carbon dioxide containing gas stream through a main heatexchange system comprising one or more heat exchangers where thetemperature of the gas stream is reduced to a point that a portion ofthe carbon dioxide in the gas stream is transformed into solid carbondioxide with the remainder of the carbon dioxide remaining in gaseousform, the carbon dioxide containing gas stream having a pressure fromabout 0.8 to 3.0 bar absolute; b. separating the solid carbon dioxidefrom the cooled carbon dioxide containing gas stream to form a stream ofsolid carbon dioxide and a cooled partially carbon dioxide depleted gasstream; c. routing the cooled partially carbon dioxide depleted gasstream back through the main heat exchange system where the cooledpartially carbon dioxide depleted gas stream is heated to a temperaturethat ranges from about −20° C. to about 50° C. to recover sensible heatand then venting the carbon dioxide depleted gas stream that iswithdrawn from the heat exchanger; and d. using the stream of solidcarbon dioxide produced to reduce the temperature of the carbon dioxidecontaining gas stream in step b) by: i) subjecting a portion of thesolid carbon dioxide to a pressure that is equal to or greater than thetriple point pressure of carbon dioxide in a first vessel incommunication with the main heat exchange system to produce a carbondioxide liquid stream and recovering fusion heat and sensible heat; ii)vaporizing a portion of the solid carbon dioxide in a second vessel incommunication with the main heat exchange system at sub-atmosphericpressure to produce a sub-atmospheric pressure carbon dioxide gas andrecovering at least sublimation heat; and iii) compressing thesub-atmospheric pressure carbon dioxide gas obtained to produce acompressed carbon dioxide stream.
 18. The process of claim 17, whereinthe carbon dioxide containing stream is compressed prior to being routedinto the main heat exchanger in order to have a pressure from about 1.1to 3.0 bar absolute.
 19. The process of claim 17, wherein the carbondioxide containing gas stream is a flue gas stream.
 20. The process ofclaim 17, wherein in step d), the vaporization of the solid carbondioxide is carried out with at least two different sub-atmosphericpressures.
 21. The process of claim 20, wherein the vaporization of thesolid carbon dioxide with at least two different sub-atmosphericpressures is performed in at least two independent vessels with eachindependent vessel corresponding to a different sub-atmosphericpressure.
 22. The process of claim 21, wherein the vessels are incommunication with the main heat exchanger through a series of pipesthat run through the main heat exchange system and the various vessels.23. The process of claim 22, wherein the recovery of the sublimationheat is carried out by circulating one or more fluids between the mainheat exchange system and the vessels to recover cold from the phasechanges in the vessels and release this cold in the main heat exchangesystem.
 24. The process of claim 23, where the one or more fluids arecirculated by pumping the one or more fluids or by thermo-siphoning. 25.The process of claim 24, wherein the one or more fluids are selectedfrom CF₄, NF₃, C₂H₆
 26. The process of claim 17, wherein the compressedcarbon dioxide stream is further treated to obtain the desired carbondioxide product.
 27. The process of claim 17, wherein the carbon dioxideliquid stream is further treated to obtain the desired carbon dioxideproduct.
 28. The process of claim 17, wherein the latent heat of fusionfor the liquefaction of the solid carbon dioxide in step D i is obtainedfrom condensation of gaseous carbon dioxide in the stream of compressedcarbon dioxide stream obtained from step D iii.